Sound loudness is a psychoacoustic correlation of the physical strength of sound. In accordance with psychoacoustic studies, when an audio of interest is heard by a headphone, an earphone or the like in presence of the background noise (competing sound), the perceived audio loudness decreases as the competing sound level increases. As used herein, the perceived loudness of an audio signal in the presence of the background noise may be referred to as “partial loudness.” FIG. 1 shows the relationship between the partial loudness and the background noise. As can be seen, as the noise signal level increases from 10 dB (curve 1) to 40 dB (curve 2), the partial loudness of an audio signal with the same signal excitation level decreases. To address this problem, there are attempts to automatically control the perceived audio loudness in the presence of the competing sound, so as to avoid the audio loudness being lower or higher than a satisfied level.
For example, it has been proposed to maintain a constant partial loudness of in presence of a noise signal. In such a solution, the loudness of the audio signal in quiet conditions (in an environment free of noise signal) is derived and set as a fixed target partial loudness. In response to the presence of the noise signal as well as the change of the noise signal level, an appropriate gain for the audio signal is derived to ensure that the partial loudness of the audio signal is maintained at the fixed target partial loudness. For example, in FIG. 1, in order to maintain the same partial loudness (0.1 sone in dB) when the noise signal level increases from 10 dB (curve 1) to 40 dB (curve 2), a required gain for the audio signal is illustrated. By applying the required gain, the audio signal is maintained at a constant partial loudness under different noise signals.
It will be appreciated by those skilled in the art that when the noise signal level changes, simply maintaining the constant partial loudness usually cannot meet the user's expectation in terms of audio loudness. That is, in this event, the user would probably not be satisfied with the constant partial loudness. Therefor there is a need in the art for a solution of more flexible and satisfied audio signal loudness control.